U.S. patent number 7,834,807 [Application Number 12/122,585] was granted by the patent office on 2010-11-16 for retro-directive ground-terminal antenna for communication with geostationary satellites in slightly inclined orbits.
This patent grant is currently assigned to Spatial Digital Systems, Inc.. Invention is credited to Donald Chin-Dong Chang.
United States Patent |
7,834,807 |
Chang |
November 16, 2010 |
Retro-directive ground-terminal antenna for communication with
geostationary satellites in slightly inclined orbits
Abstract
A retro-directive antenna for communicating with a geostationary
satellite autonomously detects the direction from which a signal is
received, and transmits a beam that points back along the same
direction. An array feed is used to illuminate a parabolic
reflector. Each feed element of the retro-directive antenna is
associated with a unique pointing direction of the beam in the far
field. As the transmit energy is switched to different feed
elements, the far-field beam is scanned, making it possible to
track a geostationary satellite in a slightly inclined orbit. This
eliminates the need for mechanical tracking and maintains high
antenna gain in the direction of the geostationary satellite. The
use of a toroidal reflector with multiple linear array feeds spaced
in the azimuth direction enables multi-beam operation, allowing
multiple geostationary satellites, spaced by up to fifteen beam
widths in azimuth, to be tracked simultaneously and
independently.
Inventors: |
Chang; Donald Chin-Dong
(Thousand Oaks, CA) |
Assignee: |
Spatial Digital Systems, Inc.
(Chatsworth, CA)
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Family
ID: |
40071914 |
Appl.
No.: |
12/122,585 |
Filed: |
May 16, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080291083 A1 |
Nov 27, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60930956 |
May 21, 2007 |
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Current U.S.
Class: |
342/370; 342/373;
342/359 |
Current CPC
Class: |
H04B
7/086 (20130101); H01Q 3/2605 (20130101); H01Q
3/24 (20130101); H01Q 3/2647 (20130101); H01Q
3/46 (20130101); H04B 7/0615 (20130101) |
Current International
Class: |
H01Q
1/00 (20060101); H01Q 3/00 (20060101) |
Field of
Search: |
;342/370,354,359,373 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tarcza; Thomas H
Assistant Examiner: Mull; Fred H
Attorney, Agent or Firm: O'Melveny & Myers LLP
Parent Case Text
RELATED APPLICATION DATA
This application claims the benefit, pursuant to 35 U.S.C.
.sctn.119(e), of U.S. provisional application Ser. No. 60/930,956,
filed May 21, 2007.
Claims
What is claimed is:
1. A retro-directive antenna terminal comprising: an antenna
reflector adapted to transmit and receive radio-frequency signals
to and from at least one satellite; a feed array comprising at
least two feed elements situated near a focus of the antenna
reflector; an input section including a receive-side
radio-frequency processor configured to generate a spatial
receive-side Fourier transform (FT) of the radio-frequency signals
received from the at least one satellite; a direction-of-arrival
(DOA) processor adapted to measure a phase profile of the signals
received from the at least one satellite after application of the
receive-side FT; a digital-beam-forming (DBF) processor adapted to
calculate beam weight vectors (BWVs) corresponding to the phase
profile and to apply them to a transmit signal; and a transmit
section including a transmit-side radio-frequency processor
configured to generate a spatial transmit-side FT of the transmit
signal and to apply the transmit signal to the feed array after
application of the transmit-side FT; wherein, by applying the BWVs
to the transmit signal and generating the transmit-side FT, the
transmit section produces a signal radiated from the feed array
that illuminates the antenna reflector to produce a beam that is
directed back along a same direction as the radio-frequency signals
received from the at least one satellite.
2. The retro-directive antenna terminal of claim 1, wherein the
input section further includes: at least two low-noise amplifiers
(LNAs) connected to corresponding ones of the at least two feed
elements; a frequency down-conversion portion adapted to
down-convert the radio-frequency signals received from the at least
one satellite after application of the receive-side FT; and a
digitizing device adapted to generate digital samples of the
signals received from the at least one satellite after frequency
down-conversion.
3. The retro-directive antenna terminal of claim 2, wherein: the
frequency down-conversion portion of the input section comprises at
least two frequency down-converters connected to outputs of the
receive-side radio-frequency processor; and the digitizing device
of the input section comprises at least two analog-to-digital
converters connected to corresponding ones of the at least two
frequency down-converters.
4. The retro-directive antenna terminal of claim 2, further
comprising: a switch matrix connected to outputs of the at least
two LNAs, and adapted to select one of the at least two feed
elements to generate a primary receive signal; and a primary
frequency down-converter adapted to down-convert the primary
receive signal.
5. The retro-directive antenna terminal of claim 1, wherein the
transmit section further includes: a complex multiplier adapted to
multiply a digital transmit signal and the BWVs calculated by the
DBF processor; a digital synthesis device adapted to generate an
analog transmit signal from the digital transmit signal after
multiplication by the BWVs; a frequency up-conversion portion
adapted to up-convert the analog transmit signal to radio
frequency; and an amplification section adapted to amplify the
analog transmit signal after up-conversion to radio frequency.
6. The retro-directive antenna terminal of claim 5, wherein: the
digital synthesis device of the transmit section comprises at least
two digital-to-analog converters; and the frequency up-conversion
portion of the transmit section comprises at least two frequency
up-converters connected to corresponding ones of the at least two
digital-to-analog converters and to inputs of the transmit-side
radio-frequency processor.
7. The retro-directive antenna terminal of claim 5, wherein the
amplification section of the transmit section comprises at least
two solid-state power amplifiers adapted to amplify the analog
transmit signal after up-conversion to radio frequency.
8. The retro-directive antenna terminal of claim 1, wherein the
receive-side radio-frequency processor comprises a receive-side
Butler Matrix comprising at least two inputs associated with
corresponding ones of the at least two array elements, and at least
two outputs.
9. The retro-directive antenna terminal of claim 1, wherein the
transmit radio-frequency processor comprises a transmit-side Butler
Matrix comprising at least two outputs connected to corresponding
ones of the at least two array elements, and at least two
inputs.
10. The retro-directive antenna terminal of claim 1, further
comprising: a code generator adapted to generate at least two
receive-side pseudonoise (PN) code outputs wherein the two
receive-side PN outputs are mutually orthogonal, and to generate at
least two transmit-side PN code outputs wherein the two
transmit-side PN outputs are mutually orthogonal; at least two
receive-side bi-phase modulators connected to outputs of the
receive-side radio-frequency processor and to corresponding ones of
the at least two receive-side PN outputs; and at least two
transmit-side bi-phase modulators connected to inputs of the
transmit-side radio-frequency processor and to corresponding ones
of the at least two transmit-side PN code outputs.
11. The retro-directive antenna terminal of claim 10, further
comprising a receive-side summing device adapted to sum outputs of
the receive-side radio-frequency processor after modulation by the
at least two receive-side bi-phase modulators.
12. The retro-directive antenna terminal of claim 11, wherein the
input section further includes: a frequency down-converter
connected to the receive-side summing device; an analog-to-digital
converter connected to the frequency down-converter; and at least
two matched filters adapted to correlate outputs of the
analog-to-digital converter with the at least two receive-side PN
code outputs of the code generator.
13. The retro-directive antenna terminal of claim 12, further
comprising a receive-side complex multiplier adapted to multiply
the BWVs calculated by the DBF processor with outputs of the at
least two matched filters to generate a primary receive signal.
14. The retro-directive antenna terminal of claim 10, wherein the
transmit section further comprises: a digital-to-analog converter;
and a frequency up-converter connected to the digital-to-analog
converter and to the at least two transmit-side bi-phase
modulators.
15. A retro-directive antenna terminal comprising: an antenna
reflector adapted to transmit and receive radio-frequency signals
to and from at least one satellite; a feed array comprising N feed
elements situated near a focus of the antenna reflector, wherein N
is a positive integer greater than one; an input section including
a receive-side Butler Matrix comprising N inputs and N outputs; a
direction-of-arrival (DOA) processor adapted to measure a phase
profile of the radio-frequency signals received from the at least
one satellite; a digital-beam-forming (DBF) processor adapted to
calculate beam weight vectors (BWVs) corresponding to the phase
profile; and a transmit section comprising: a complex multiplier
adapted to multiply a digital transmit signal and the BWVs
calculated by the DBF processor; and a transmit-side Butler Matrix
comprising N inputs and N outputs, wherein the N outputs are
connected to corresponding ones of the N feed elements.
16. The retro-directive antenna of claim 15, wherein the input
section further includes: N low-noise amplifiers (LNAs) connected
to corresponding ones of the N feed elements; N frequency
down-converters connected to corresponding ones of the N outputs of
the receive-side Butler Matrix; and N analog-to-digital converters
connected to corresponding ones of the N frequency down-converters
and adapted to generate digital samples of signals from the N
frequency down-converters.
17. The retro-directive antenna terminal of claim 16, further
comprising: a switch matrix connected to outputs of the N LNAs, and
adapted to select one of the N feed elements to generate a primary
receive signal; and a primary frequency down-converter adapted to
down-convert the primary receive signal.
18. The retro-directive antenna of claim 15, wherein the transmit
section further includes: N digital-to-analog converters adapted to
synthesize N analog transmit signals from the digital transmit
signal after multiplication by the BWVs; N frequency up-converters
adapted to up-convert corresponding ones of the N analog transmit
signals to radio frequency; and an amplification section adapted to
amplify the N analog transmit signals after up-conversion to radio
frequency.
19. The retro-directive antenna terminal of claim 18, wherein the
amplification section of the transmit section comprises N
solid-state power amplifiers adapted to amplify corresponding ones
of the N analog transmit signals after up-conversion to
radio-frequency.
20. A retro-directive antenna terminal comprising: an antenna
reflector adapted to transmit and receive radio-frequency signals
to and from at least one satellite; a feed array comprising N feed
elements situated near a focus of the antenna reflector, wherein N
is a positive integer greater than one; an input section
comprising: a receive-side Butler Matrix comprising N inputs and N
outputs; and a code generator adapted to generate N mutually
orthogonal receive-side pseudonoise (PN) sequences and N mutually
orthogonal transmit-side PN sequences, wherein signals from the N
feed elements are modulated by corresponding ones of the N
receive-side PN sequences; a direction-of-arrival (DOA) processor
adapted to measure a phase profile of the signals received from the
at least one satellite; a digital-beam-forming (DBF) processor
adapted to calculate beam weight vectors (BWVs) corresponding to
the phase profile; and a transmit section including a transmit-side
Butler Matrix comprising N inputs and N outputs, wherein the N
outputs are connected to corresponding ones of the N feed
elements.
21. The retro-directive antenna of claim 20, wherein the input
section further includes: N low-noise amplifiers (LNAs) connected
to corresponding ones of the N feed elements; N receive-side
bi-phase modulators driven by corresponding ones of the N
receive-side PN sequences and connected to corresponding ones of
the N outputs of the receive-side Butler Matrix; a summing device
adapted to sum outputs of the N receive-side bi-phase modulators; a
frequency down-converter adapted to down-convert an output of the
summing device; an analog-to-digital converter adapted to generate
digital samples of an output of the frequency down-converter; and N
matched filters adapted to correlate the digital samples with each
of the N receive-side PN sequences.
22. The retro-directive antenna terminal of claim 21, further
comprising a receive-side complex multiplier adapted to multiply
the BWVs calculated by the DBF processor with outputs of the N
matched filters to generate a primary receive signal.
23. The retro-directive antenna of claim 20, wherein the transmit
section further comprises: a complex multiplier adapted to multiply
a digital transmit signal and the BWVs calculated by the DBF
processor; a digital-to-analog converter adapted to synthesize an
analog transmit signal from the digital transmit signal after
multiplication by the BWVs; a frequency up-converter adapted to
up-convert the analog transmit signal to radio frequency; N
transmit-side bi-phase modulators driven by corresponding ones of
the N transmit-side PN sequences and connected to an output of the
frequency up-converter and adapted to generate N modulated
radio-frequency transmit signals; and an amplification section
adapted to amplify the N modulated radio-frequency transmit
signals.
24. The retro-directive antenna terminal of claim 23, wherein the
amplification section of the transmit section comprises N
solid-state power amplifiers adapted to amplify corresponding ones
of the N modulated radio-frequency transmit signals.
25. A retro-directive antenna terminal comprising: an antenna
reflector adapted to transmit and receive radio-frequency signals
to and from M satellites, wherein M is a positive integer; a feed
array comprising M linear feed arrays, wherein each of the M linear
feed arrays comprises N feed elements, wherein N is a positive
integer greater than one; an input section including M receive-side
Butler Matrices each comprising N inputs and N outputs; M
direction-of-arrival (DOA) processors adapted to measure a phase
profile of each of the radio-frequency signals received from each
of the M satellites; M digital-beam-forming (DBF) processors
adapted to calculate M sets of beam weight vectors (BWVs)
corresponding to the M phase profiles of corresponding ones of the
M satellites; and a transmit section comprising: M complex
multipliers adapted to multiply M digital transmit signals and the
M sets of BWVs calculated by the M DBF processors; and M
transmit-side Butler Matrices each comprising N inputs and N
outputs, wherein the N outputs of each of the M transmit-side
Butler Matrices are connected to corresponding ones of the N feed
elements of each of the M linear feed arrays; wherein, each of the
M linear feed arrays is adapted to create a beam pointing in a
direction to corresponding ones of the M satellites.
26. The retro-directive antenna terminal of claim 25, wherein the
antenna reflector has a shape that is substantially parabolic in an
elevation direction and substantially circular in an azimuth
direction.
27. In a system including an antenna terminal comprising a feed
array including N feed elements, wherein N is a positive integer
greater than one, and adapted to receive an incoming
radio-frequency beam from a satellite and to transmit an outgoing
radio-frequency beam to the satellite, a method of measuring the
direction-of-arrival of the incoming radio-frequency beam and
transmitting the outgoing radio-frequency beam back along the same
direction comprises: performing a spatial Fourier transform (FT) of
the incoming radio frequency beam received by the N feed elements
to produce a transformed signal; frequency down-converting the
transformed signal to produce a baseband signal; digitizing the
baseband signal; measuring a phase profile of the baseband signal
after digitizing; calculating beam weight vectors (BWVs)
corresponding to the phase profile; multiplying a digital transmit
signal with the BWVs; synthesizing analog transmit signals from the
digital transmit signals after multiplication; frequency
up-converting the analog transmit signals to radio frequency;
amplifying the analog transmit signals after up-conversion;
performing a spatial FT of the analog transmit signals after
up-conversion and amplification; and applying the analog transmit
signals after application of the FT to the N feed elements of the
feed array to produce the outgoing radio-frequency beam.
28. The method of claim 27, wherein the step of performing a
spatial FT of the incoming radio frequency beam comprises routing
signals from the N feed elements through a Butler Matrix having N
inputs.
29. The method of claim 27, wherein the step of performing a
spatial FT of the analog transmit signals after up-conversion and
amplification comprises routing the analog transmit signals through
a Butler Matrix having N inputs.
30. The method of claim 27, wherein the steps of performing a
spatial FT of the incoming radio frequency beam to produce a
transformed signal and frequency down-converting the transformed
signal to produce a baseband signal further comprise: modulating
the transformed signal with N mutually orthogonal pseudonoise (PN)
codes to produce N PN-modulated signals; summing the N PN-modulated
signals to produce a composite signal; digitizing the composite
signal; and correlating the composite signal after digitization
with the N mutually orthogonal PN codes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ground-terminal antennas for
communicating with satellites in geostationary orbit. More
particularly, it relates to low-cost, electronically steerable
antennas adapted to compensate for motion of a satellite with
respect to its fiducial geostationary position, and to
electronically steerable multi-beam antennas adapted to compensate
for motions of multiple satellites simultaneously.
2. Description of Related Art
Satellites in geostationary orbit are widely used for
communications and broadcast applications. When the orbit of a
satellite lies along a path 35,786 km directly over the equator,
its orbital velocity exactly matches the rate of rotation of the
Earth, and the satellite remains fixed in the sky relative to an
observer on the ground. This greatly simplifies the design of
ground terminals because they can be designed to point in a single
fixed direction and do not require bulky motorized gimbals or
tracking hardware. However, while a satellite in geostationary
orbit should theoretically remain at a fixed location in the sky,
perturbations to its orbit caused by interactions of the Sun and
Moon as well as the non-spherical shape of the Earth itself cause
the orbit of the satellite to drift away from its fiducial
geostationary point. As shown in FIGS. 1A-C, a satellite that
drifts into a slightly inclined orbit with respect to the equator
begins to trace out an elongated figure-eight pattern oriented in
the north-south direction in the sky, as seen by the observer on
the ground. This motion can result in severe loss of signal by a
ground terminal with a simple fixed antenna. A number of methods to
address this problem have been developed, but all have significant
drawbacks.
One method of addressing this problem is to articulate the ground
terminal by adding gimbals and a mechanical tracking system to
allow the antenna pointing to be continually adjusted in order to
track the satellite. However, such a solution adds significant
cost, bulk, and complexity and is not suitable for applications
requiring a large number of ground stations, such as
direct-broadcast television.
Another method is to selectively broaden the antenna pattern in the
north-south direction to account for the increased satellite motion
in this direction. For example, a typical one-meter-diameter
parabolic antenna operating at Ku band will exhibit a beam width of
approximately two degrees. If the antenna reflector is compressed
into an ellipse, the pattern in the north-south direction can be
stretched to twelve-to-fourteen degrees, covering excursions of a
satellite in an orbit inclined up to six or seven degrees with
respect to the equator. However, stretching the radiation pattern
significantly reduces antenna gain, negatively impacting receive
performance and requiring additional power for transmit.
Another method is to actively control the position of the satellite
by firing thrusters to perform "station-keeping" maneuvers in order
to keep the satellite as close as possible to the equator to
minimize north-south excursions. The tighter the station-keeping
requirements imposed by the capabilities of the ground terminals,
the more frequent are the required station-keeping maneuvers. When
the satellite runs out of fuel, it can no longer be maintained in
geostationary orbit, so the frequency of such maneuvers directly
affects the useful life of the satellite.
Thus, it would be useful to provide a design for a low-cost,
compact, ground terminal that does not require mechanical tracking
and that would enable a relaxation of tight station-keeping
requirements for geostationary satellites in order to reduce fuel
consumption and prolong their useful lifetimes.
SUMMARY OF THE INVENTION
A system is provided that autonomously detects a direction of
arrival of a signal from a geostationary satellite and generates a
transmit signal that is sent along the same direction back toward
the satellite. The system maintains high gain in the direction of
the satellite and tracks its motion without the need for a
mechanical pointing system.
An embodiment of a retro-directive antenna terminal in accordance
with the present invention includes a parabolic reflector with an
array feed positioned near its focus. The array feed includes N
feed elements, where N is an integer greater than or equal to two.
An embodiment described herein has N equal to four, providing a
compromise between the complexity of the array feed and the
pointing resolution of the antenna beam. However, an array feed
comprising as few as two elements or more than four elements would
also fall within the scope and spirit of the present invention.
A parabolic reflector typically has a limited scan range, and
far-field beams arriving from directions that are a few degrees off
of boresight will focus at locations that are slightly offset from
the boresight focus of the antenna. Thus, energy arriving from
off-boresight angles will preferentially illuminate elements of the
array feed that are positioned slightly away from the reflector
focus. Similarly, energy radiated from feed array elements that are
located slightly off focus will result in far-field beams that are
pointed in directions a few degrees off of boresight. Thus, for a
fixed boresight direction, an array feed allows for electronic
scanning of the antenna beam within the limited scan range of the
parabolic reflector. In the case of geostationary satellites that
are inclined by a few degrees from the equator and thus move in a
north-south direction relative to the ground terminal during the
course of each day, a feed array oriented in the north-south, or
elevation direction will allow the motion of the satellite to be
tracked without mechanically moving the boresight pointing
direction.
An enhanced scan range in the azimuth direction can be achieved
with an antenna reflector having a circular profile. Thus, a
parabolic toroidal reflector having a parabolic profile in
elevation and a circular profile in azimuth will exhibit a moderate
scan range in elevation, as described above, combined with a wider
scan range in azimuth. Such an antenna, equipped with an
appropriate feed array, would be able to simultaneously track
multiple geostationary satellites separated in azimuth by over ten
beam widths.
In an embodiment of an antenna terminal in accordance with the
present invention, signals arriving at the N array feed elements
are individually amplified by low-noise amplifiers (LNAs) and
divided into two paths: a main receive path and a diagnostic path.
The signals in the diagnostic path are applied to the inputs of an
N-by-N Butler Matrix (BM) or other device configured to perform a
spatial Fourier transform (FT) of the array feed signals. Various
inputs of the BM generate different phase progressions among the N
outputs. The outputs of the Butler Matrix are then frequency
down-converted to form N baseband signals that are each digitized
by analog-to-digital converters. A direction-of-arrival processor
then measures the phase slope of the digitized signals to determine
the direction of the wavefront incident on the feed array elements
and thus, the direction of arrival of the signal from the
satellite. This information also enables the system to determine
which of the feed array elements is being illuminated by the signal
arriving from the satellite.
In the main receive path, the outputs of the LNAs are routed to a
switch matrix that is switched to select the illuminated feed array
element as the primary receive signal of the system. This signal
may be frequency down-converted and sent to the primary receiver of
the system, which might be a digital television receiver or other
communications device.
A digital beam forming (DBF) processor uses the measured phase
slope information to calculate receive beam weight vectors (BWVs),
which are sets of complex coefficients that can be used to adjust
the amplitude and phase of the signals from the elements of an
array in order to create coherent beams pointing in selected
directions. The receive BWVs operate to index to proper transmit
BWVs that are used to create a transmit beam that will be directed
back along the direction of the receive beam. Note that the
correlation index of the receive and transmit BWVs is generated off
line and beforehand as a look-up table to assure that the transmit
and receive beams are always directed to and from the same feed
element and thus pointed in the same direction.
Digital waveforms comprising the desired transmit signals to be
sent to the satellite are multiplied by the BWVs calculated by the
DBF processor in order to create a set of N digital signals that
exhibit a phase slope that is conjugate to that of the received
signals. These N digital signals are then routed through N
digital-to-analog converters to synthesize N analog baseband
waveforms containing the transmit data and exhibiting the proper
conjugate phase slope. The N analog baseband waveforms are then
frequency up-converted to N radio-frequency signals. These
radio-frequency signals are amplified by solid-state power
amplifiers or other radio-frequency amplifiers known in the art and
are applied to a transmit-side Butler Matrix, or other device
capable of performing a spatial FT. The outputs of the
transmit-side Butler matrix are then applied to the feed array
elements through diplexers, producing a transmit beam that is
directed back along the line of sight to the satellite.
In general, the receive beam can be thought of as being focused by
the parabolic reflector onto one of the elements of the array feed.
The spatial FT then produces a set of signals encoding a phase
slope that is indicative of the direction of the wavefront causing
illumination of that array element. By encoding the conjugate of
that phase slope into the transmit signal and running it through a
transmit-side FT, the transmit energy appears preferentially at
only one of the elements of the feed array. This then produces a
beam that is retro-directed back along the same line of sight as
the received beam. Of course, it is also possible that the received
beam will illuminate two of the elements of the feed array,
indicating an arrival angle between those that would illuminate a
single element. This would simply result in the transmit signal's
also being applied to the same two array feed elements to produce a
retro-reflected beam.
Thus in the preferred embodiment, the beam formation is performed
in a "wavefront domain." The conversions from and to the beam
domain take place in two spatial FT devices (the BMs). The phase
progressions in the wavefront domain uniquely identify discrete
signal directions associated with individual antenna feed elements.
The reflector having multiple feeds is characterized as a
multi-beam antenna (MBA), and each of the feed elements corresponds
to a unique beam position in the far field. For a reflector with N
feed elements, there are N distinct far-field beam positions with
associated beam widths. After processing by the spatial FT device,
each of the N output ports receives signals from all of the N array
feed elements simultaneously. The N output ports share the same
field of view but have unique phase slopes associated with the
directions of arrival, similar to the characteristics of an array
antenna.
It is also possible to implement retro-directive antennas in the
"beam domain" without the use of spatial FT devices. As compared to
the wavefront-domain implementation described above that features
graceful degradation in the transmit beam having N power amplifiers
driving a spatial FT processor, the beam-domain implementation
would feature a one-to-N switch matrix with a single large power
amplifier at the input side.
In an alternative embodiment of a retro-directed antenna terminal
in accordance with the present invention, orthogonal coding is used
to simplify the radio-frequency hardware and to eliminate the need
for multiple down- and up-conversion stages and multiple A/D and
D/A converters. In this alternative embodiment, the received
signals from four array feed elements are routed through a Butler
Matrix or other radio-frequency FT processor. The four transformed
outputs are then routed to four bi-phase modulators, each of which
is driven by a separate mutually orthogonal pseudonoise (PN) code
produced by a code generator. The four modulated beams are then
summed, and the composite beam is frequency down-converted by a
single down-converter and digitized by a single A/D converter. The
sampled composite beam is then routed through four matched filters,
each of which correlates the sample stream with one of the PN
sequences. Because of the mutual orthogonality of the PN sequences,
only that portion of the composite beam that was originally
modulated with the corresponding PN sequence will be recovered by
the matched filter. Thus, this process allows for the recovery of
four separate digital data streams while requiring only one
down-conversion chain and one A/D. This not only reduces RF parts
count and complexity but also eliminates problems of poorly matched
analog channels that can degrade performance.
The digital samples are processed by a direction-of-arrival
processor as before, and a DBF processor again calculates BWVs
corresponding to the measured phase slope and direction of arrival.
In addition to using the BWVs to perform transmit-side beam
forming, the BWVs are also used to multiply the receive data
streams coming out of the matched filters. This operation recovers
the coherent sum of the feed array elements for the direction of
arrival of the wavefront from the satellite and thus is a fully
digitized primary receiver signal that can be routed to the primary
receiver of the system, such as a digital television receiver or
similar device. Thus, the need for a separate analog switch matrix
and separate down-conversion chain is also eliminated.
On the transmit side, the primary digital transmit waveform is
multiplied by the complex BWVs to create a phase profile that is
conjugate to that of the receive side. The composite digital signal
is then routed to a single D/A converter that synthesizes an analog
transmit waveform encoded with the desired phase profile. This
analog waveform is then frequency up-converted to radio frequency.
The up-converted RF signal is then applied to four bi-phase
modulators driven by the same mutually orthogonal PN codes in order
to create four separate modulated RF signals. These signals are
then amplified by solid-state power amplifiers or other RF
amplification devices and are applied to the four inputs of a
transmit-side Butler Matrix or other RF FT processor. The
constructive and destructive combinations that are formed inside
the Butler Matrix then result in the output's being directed to the
same feed element or elements of the array feed that were
illuminated by the receive signal from the satellite.
The foregoing discussion described an embodiment of a
retro-directive antenna terminal having a four-element feed array.
However, other numbers of feed elements in the antenna feed array
are possible with corresponding adjustments to the number of inputs
to the receive-side and transmit-side Butler Matrices and other
channel-specific hardware. Such systems would also fall within the
scope and spirit of the present invention.
From the foregoing discussion, it is clear that certain advantages
have been achieved for a retro-directive antenna terminal that
autonomously detects a direction of arrival of a satellite signal
and transmits back along the same direction. Further advantages and
applications of the invention will become clear to those skilled in
the art by examination of the following detailed description of the
preferred embodiment.
Reference will be made to the attached sheets of drawing that will
first be described briefly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-C depict the orbital motion of a typical geostationary
satellite around the Earth;
FIG. 2 illustrates a block diagram of an embodiment of a
multiple-beam retro-directive ground terminal in accordance with
the present invention;
FIG. 3A and 3B is a schematic diagram illustrating grouping of
antenna feed elements to achieve finer pointing resolution;
FIG. 4 illustrates a block diagram of an alternative embodiment of
a multiple-beam retro-directive ground terminal in accordance with
the present invention;
FIGS. 5A and 5B are schematic drawings of two embodiments of
antennas in accordance with the present invention showing single
and multiple satellite-tracking capability; and
FIGS. 6A and 6B depict the azimuthal scanning capability of a
parabolic and a parabolic-toroidal antenna reflector in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention provides a simple, low-cost, limited-scan-angle,
retro-directive antenna featuring an array feed capable of steering
the antenna pattern to track orbital excursions of a geostationary
satellite in an orbit inclined with respect to the equator by
several degrees. In the detailed description that follows, like
element numerals are used to indicate like elements appearing in
one or more of the figures.
FIGS. 1A-C depict the motion of a typical geostationary satellite
106 in orbit around the Earth 102. The ideal geostationary orbit
108 lies directly above the Earth's equator and results in the
satellite's appearing stationary in the sky with respect to an
observer on the ground. Due to gravitational perturbations, the
actual orbit 104 of the satellite drifts, becoming inclined by up
to several degrees with respect to the equator. Periodic
station-keeping maneuvers are undertaken to bring the actual orbit
back toward an inclination of zero degrees. The inclined orbit 104
crosses the equatorial plane at nodes 110. FIG. 1A depicts the
orbit from a direction along a line connecting the two nodes 110.
FIG. 1B depicts the orbit from a direction perpendicular to a line
connecting the two nodes 110.
FIG. 1C depicts the apparent motion of the satellite 106 as viewed
from the ground during the course of one day. The satellite 106
traces out a figure eight in a north-south direction, appearing at
location 112 as it passes the orbital nodes 110. The height of the
figure eight depends on the inclination of the orbit 104 with
respect to the equator. The majority of the satellite displacement
is in the elevation direction; the magnitude of the displacement in
azimuth is generally an order of magnitude smaller.
FIG. 2 depicts a block diagram of an embodiment of a ground
terminal in accordance with the present invention. The terminal
includes a parabolic reflector 202 that is illuminated by four
feeds 204, which may be horns, patches, or any other types of
antenna feeds known in the art. The feeds are oriented to have the
optimal polarization response individually, and they are positioned
linearly in the focal plane along a line parallel to the local
elevation direction. The scan range of a typical parabolic
reflector is approximately +/-5 beam widths. For a one-meter
reflector at Ku band, the beam width is approximately two degrees,
and the scan capability is approximately +/-10 degrees. Signals
arriving from directions within this scan range will be focused at
slightly varying locations. Conversely, feeding the antenna from
locations that vary slightly will result in antenna beams that
point in slightly different directions within the scan range of the
reflector. Thus, feeding the antenna from different elements or
combinations of different elements of the feed array will result in
beam steering in the far field. Although FIG. 2 depicts a system
having four feeds, other systems are possible that include N feeds,
where N is an integer greater than or equal to two, and such
systems would also fall within the scope and spirit of the present
invention.
Satellite signals impinging on the reflector 202 are focused onto
the feeds 204, are amplified by low-noise amplifiers 138, and are
routed to a four-by-four Butler Matrix (BM) 212. The four-by-four
BM includes four 90-degree hybrids and two fixed phase shifters
configured in a manner well known in the art to produce an output
that is a spatial Fourier transform (FT) of the input. The BM
converts the beam-space signals from the feed elements to
wavefront-domain signals. The four output wavefront signals from
the BM are orthogonal to one another. Of course, for systems
including N feed elements, correspondingly sized N-by-N BMs would
be used. The outputs of the BM 212 are then routed through
band-pass filters 214 and then to frequency down-converters 216
that convert the radio-frequency inputs to an intermediate or
baseband frequency. The frequency-down-converted signals are then
digitized using analog-to-digital converters 218, and the digital
samples are passed to a correlation processor 220. The correlation
processor compares the digitized samples from each of the BM
outputs and calculates a phase slope across the outputs. The
direction-of-arrival (DOA) processor 222 uses this phase slope to
determine which of the antenna feeds within the feed array is being
illuminated by the signal from the parabolic or parabolic-toroidal
dish 202. This information is then used in the main receive signal
path to select the appropriate states of switches in the switch
matrix 206 in order to route the received signal from a selected
feed to the primary frequency down-converter 208 in order to
prepare the intermediate-frequency receive signal 210 that is
routed to the main receiver (not shown). Methods of forming a
spatial FT of the input other than using a Butler Matrix in the
diagnostic path may also be used and would fall within the scope
and spirit of the present invention.
The direction-of-arrival information is also used by a digital beam
forming (DBF) processor 224 to calculate an appropriate set of beam
weight vectors (BWVs) that can be applied to the main transmit
signal 230 in order to select a phase slope that is conjugate to
that of the received signal. When this phase slope is applied 228
to the transmitted beam, it results in retro-directed transmit beam
that propagates back along the direction from which the received
beam arrives. The main digital transmit signal is multiplied 228
with the BWVs generated by the DBF processor 224, and the composite
waveform is synthesized using digital-to-analog converters 226. The
synthesized baseband waveform is then frequency up-converted 232
and amplified 234 and applied to a transmit-side Butler Matrix 236.
The outputs of the transmit-side BM are then applied through
diplexers to the antenna feeds 204 which illuminate the reflector
202 and produce a retro-directed far-field beam. Note that the
proper selection of the BWVs applied to the transmit signal 230 by
the DBF processor 224 results in constructive and destructive
combining through the transmit BM 236 to result in a non-zero
output at only one of the antenna feed elements 204--the same one
upon which the receive signal is incident. In other words, the
selection of a set of BWVs at digital baseband performs a switching
function, directing RF energy to the selected antenna feed
element.
In the embodiment discussed above, scanning of the far-field beam
may be performed in four discrete beam positions, each position
corresponding to one of the four feed element locations. However,
because a BM is a linear device, it is also possible to vary the
signal intensity across multiple feed elements to provide finer
scanning resolution. For example, FIGS. 3A and 3B depict possible
groupings of adjacent antenna feed array elements that may be used
to point the far-field beam in directions between those achieved by
using a single feed element. FIG. 3A depicts an embodiment of a
four-element array in accordance with the present invention. The
antenna feed elements 302, 304, 306, and 308 may be driven one at a
time in order to point the far-field beam in four slightly
different directions. Alternatively, elements 302 and 304 can be
driven together as indicated at 310 by applying linear combinations
of BWVs to the digital baseband transmit signal that result in
driving element 302 and element 304. The resulting far field beam
will point in a direction between the beams formed when either
element 302 or 304 is driven alone. Similarly, other adjacent
combinations may be formed, such as those indicated at 312 and
314.
FIG. 3B depicts an alternative embodiment of a feed array in
accordance with the present invention in which nine antenna feed
array elements, 320-336, are used. Similarly, combinations of
adjacent elements, e.g., 340, 348, may be used to provide finer
resolution than driving individual elements alone would achieve.
Systems using N array elements, where N is an integer greater than
or equal to two, would also fall within the scope and spirit of the
present invention.
Although the above discussion focused on the transmit-side
application of the feed array, the concept of grouping adjacent
elements to increase the pointing resolution is equally effective
for the receive operation. Again, because the BM 212 is a linear
device, a signal incident on the parabolic reflector 202 that
illuminates more than one feed element, e.g., the combination 310,
can be viewed as a linear combination of a signal that illuminates
element 302 and one that illuminates element 304. From this linear
combination, the DOA processor 222 is able to determine a direction
of arrival that lies between those of each element taken
individually.
The far-field radiation produced by the feed arrays depicted in
FIGS. 3A and 3B are linearly polarized. However, the techniques
described above are equally applicable to circularly polarized
radiation. If a polarizing device, such as one implemented using
meander-line techniques well known in the art, is placed in front
of the feed array, transmitted linearly polarized radiation can be
circularly polarized. Similarly, received circularly polarized
radiation can be converted to linearly polarized radiation before
being collected by the feed-array elements.
FIG. 4 illustrates an alternative embodiment of retro-directive
terminal in accordance with the present invention. This embodiment
takes advantage of high-speed digital electronics to simplify the
radio-frequency processing. Signals impinging on a parabolic
reflector 402 are focused onto an array feed 404. The detected
power from each feed element 404 is routed through a low-noise
amplifier 406 and sent to a four-input BM 408. It should be
appreciated that systems with more or fewer array feed elements and
corresponding BM inputs and outputs would also fall within the
scope and spirit of the present invention. Each output of the BM
408 is then bi-phase modulated 410 by a pseudonoise (PN) code
sequence generated by a code generator 430. Each output of the code
generator 430 is used to modulate a corresponding one of the
outputs of the BM 408, and the PN code sequences are mutually
orthogonal. The modulated outputs of the BM are then summed
together 412, and the composite RF signal is frequency
down-converted 413 and then digitized using an analog-to-digital
converter 414. As compared to the embodiment described with
reference to FIG. 2, above, four individual down-conversion devices
(e.g., 216) are consolidated into a single down-converter 413,
which allows for better channel matching and simplification of the
radio-frequency portion of the circuit, assuming the processing
power of the digital circuit is adequate. Also eliminated from the
embodiment of FIG. 2 is a separate analog receive path including a
switch matrix 206 and an independent frequency down-converter 208
for producing the main receive signal channel. As the speed of
digital processing hardware increases and the cost decreases,
systems will tend to move further toward the digital architecture
depicted in FIG. 4.
The digitized samples from the A/D 414 are then passed through a
set of matched filters that correlate the samples with each of the
orthogonal codes applied to the outputs of the receive BM 408.
Because of the mutual orthogonality of the PN code sequences,
digital samples corresponding to the four outputs of the BM are
recovered. A direction-of-arrival (DOA) processor 422 analyzes the
four digitized outputs of the BM 408 and calculates a phase slope
that enables calculation of the direction of arrival of the input
radio-frequency beam. A set of beam weight vectors (BWVs) are
calculated by a digital beam forming (DBF) processor 420 to
correspond to this direction of arrival. These directional weights
are then applied 418 to the outputs of the matched filter 416 to
produce the digital receive signal 450 that is sent off to the main
system receiver.
The main digital transmit signal 426 of the system is also
multiplied 424 by a corresponding set of BWVs calculated by the DBF
processor 420 to produce a phase slope that is conjugate to the
phase slope of the received beam. The transmit signals, mixed with
appropriate BWVs are then digitally summed 428, and a baseband
waveform is synthesized using a digital-to-analog (D/A) converter
432. The baseband waveform is frequency up-converted 434 to radio
frequency and is then modulated 436 by the same set of four
orthogonal PN codes 430 to produce four component signals that are
then filtered by band-pass filters 438, amplified 440 and applied
to the inputs of a transmit-side BM 442. The outputs of the
transmit-side BM 442 then drive the array feed elements 404 through
diplexers 444. The proper choice of BWVs applied to the transmit
signal produces inputs to the BM that are then combined in such a
way that, in general, only one output of the BM is non zero.
Of course, as described with reference to the embodiment pictured
in FIG. 2, it is also possible to group antenna feed elements to
improve the scan resolution, and in that case, more than one of the
outputs of the transmit-side BM 442 could be non zero. The matching
of the phase slopes achieved by the DOA processor 422 and the DBF
processor 420 thus enables the system transmit signals to be
retro-directed with respect to the received signals.
It should be appreciated that the systems described with reference
to FIGS. 2 and 4 do not require a continuous receive signal in
order to determine how to point the transmit beam. Both systems can
save the direction-of-arrival information calculated by the DOA
processor, e.g., 422, and use it to apply appropriate BWVs at a
later time to the transmit data stream.
FIG. 5A depicts a schematic view of an embodiment of a parabolic
antenna in accordance with the present invention. The reflector 502
has a paraboloid surface and is illuminated by a linear feed array
504 comprising four feed element aligned in the local elevation
(north-south) direction. The beam from the satellite is indicated
schematically at 506. By switching the transmit drive signal to
various elements of the feed 504 as described previously, the beam
can be made to scan in the elevation direction as indicated at
508.
In another embodiment in accordance with the present invention and
illustrated in FIG. 5B, the reflector has a parabolic-toroidal
surface that is parabolic in the elevation direction and circular
in the azimuth direction. The feed 530 of this embodiment comprises
four independent four-element linear arrays, e.g., 522 and 524.
Each of the four-element arrays is positioned in the focal plane
along a line in the azimuth direction. The beams created by each of
the four four-element feed arrays are shown schematically, e.g.,
526 and 528. The displacement of each feed array along the azimuth
direction creates a beam that is deflected in azimuth from the
boresight of the antenna 520. Each individual beam can also be
scanned in the elevation dimension, e.g., 532, by controlling which
element of the linear array 524 is driven. Thus, such a system
effectively combines four elevation tracking stations into a single
aperture and could be used to track four independent geostationary
satellites in slightly inclined orbits as long as they were not
spaced too far apart in azimuth.
FIGS. 6A and 6B illustrate the improved azimuthal scanning
performance of a parabolic-toroidal antenna over a parabolic
antenna. FIG. 6A depicts azimuth cuts of the antenna pattern of a
parabolic antenna. Degrees off of boresight are plotted along the
horizontal axis 608, and the relative pattern intensity in dBi is
plotted along the vertical axis 606. Individual azimuth cuts, e.g.,
604, are plotted as a function of boresight angle. The figure
illustrates that the pattern of a parabolic antenna falls off by 5
dB at a scan angle of 25 degrees.
Toroidal reflectors, on the other hand, feature better scanning
characteristics in azimuth than parabolic reflectors. It is
possible to design toroidal reflectors having a scan range in
azimuth of +/-10 to +/-15 beam widths. FIG. 6B illustrates the same
azimuth cuts for a parabolic-toroidal antenna with a circular shape
in the azimuth dimension. The pattern cuts, e.g., 612, are plotted
as a function of boresight angle 616. As is evident from the
figure, the amplitude falls off by only about 1 dB at scan angles
of 25 degrees, illustrating the improved scanning performance of
the toroidal reflector.
Thus, a retro-directive antenna is achieved that takes advantage of
the limited field-of-view presented by a parabolic reflector fed by
an array feed. Each feed element of the retro-directive antenna is
associated with a unique elevation pointing direction of the beam
in the far field. As the transmit energy is switched to different
feed elements, the far-field beam is scanned in elevation, making
it possible to track a geostationary satellite in a slightly
inclined orbit. The retro-directive antenna is able to autonomously
detect the elevation direction from which a signal is received, and
a direction-of-arrival processor and a digital beam-forming
processor are used to prepare a transmit beam that points back
along the same direction. This eliminates the need for mechanical
tracking and maintains high antenna gain in the direction of the
geostationary satellite.
A similar technique is applied in parallel in the azimuth direction
to create a multi-beam retro-directive antenna that can track
multiple geostationary satellites simultaneously and independently.
A parabolic-toroidal reflector is preferentially coupled to an
array feed comprising multiple linear arrays, each of which is
capable of supporting tracking in the elevation direction. The
displacement of the multiple linear arrays in the azimuth direction
creates independent simultaneous beams that point in different
azimuth directions, each capable of independently tracking motion
in the elevation direction. Those skilled in the art will likely
recognize further advantages of the present invention, and it
should be appreciated that various modifications, adaptations, and
alternative embodiments thereof may be made within the scope and
spirit of the present invention. The invention is further defined
by the following claims.
* * * * *